1. Field of the Invention
High temperature alloys with high oxidation and corrosion resistance on the basis of intermetallic compounds which are suitable for directional solidification and supplement the conventional nickel-based superalloys.
The invention relates to the further development and improvement of the alloys based on the intermetallic compound Ni.sub.3 Al, having further additions increasing the thermal stability and the oxidation resistance.
In particular it relates to an oxidation-resistant and corrosion-resistant high-temperature alloy for directional solidification on the basis of an intermetallic compound of the nickel aluminide type.
2. Discussion of Background
The intermetallic compound Ni.sub.3 Al has some interesting properties which make it appear attractive as a structural material in the average temperature range. This includes, inter alia, its low density compared with superalloys. However, its brittleness and its inadequate corrosion resistance stand in the way of its technical usability. The former can certainly be improved by additions of boron, in which case higher strength values are also achieved (cf. C.T.Liu et al, "Nickel Aluminides for structural use", Journal of Metals, May 1986, pp. 19-21). Nonetheless, this method, even while using high cooling rates, has not lead to any results which are useful in practice in the production of strip.
The corrosion resistance and oxidation resistance of alloys of this type based on Ni.sub.3 Al can be improved by additions of silicon or chromium (cf. M. W. Grunling and R.Bauer, "The role of Silicon in corrosion resistant high temperature coatings", Thin Films, Vol. 95, 1982, pp. 3-20). In general, alloying with silicon is the more practicable method than that with chromium, since the intermetallic compound Ni.sub.3 Si appearing at the same time can be fully mixed in Ni.sub.3 Al. This concerns isomorphous states, where no further undesirable phases are formed (cf. Shouichi Ochiai et al, "Alloying behaviour of Ni.sub.3 Al, Ni.sub.3 Ga, Ni.sub.3 Si and Ni.sub.3 Ge", Acta Met. Vol. 32, No. 2, pp. 289, 1984).
However, the thermal stability of Ni.sub.3 Al as well as of the above modified alloys is still inadequate, as follows from publications on intermetallic compounds (cf. N.S.Stoloff, "Ordered alloys-physical metallurgy and structural applications", International metals reviews, Vol. 29, No. 3, 1984, pp. 123-135).
It is known that, inter alia, silicon increases the corrosion resistance and oxidation resistance of surface layers forming protective oxides in coatings of high temperature alloys. This has been the subject of extensive investigations (cf. F. Fitzer and J. Schlichting, "Coatings containing chromium, aluminium, and silicon for high temperature alloys", High temperature corrosion, National association of corrosion engineers, Houston Texas, San Diego, Calif., Mar. 2-6, 1981, pp. 604-614).
In general, the properties of these known modified Ni.sub.3 Al materials still do not meet the technical requirements in order to manufacture useful work pieces therefrom. This especially applies with regard to thermal stability and high-temperature corrosion resistance (resistance to sulfidation). There is therefore a need for materials of this type to be further developed and improved.